Diffuse Reflectance Spectroscopy

This chapter describes available laboratory methods for recording diffuse reflectance spectra for soil materials and ways to handle the information to identify and characterize soil minerals. It considers the visible and narrow vicinal ultraviolet (UV) and infrared (IR) regions of the spectrum. The chapter first provides a brief introduction to the theoretical principles of diffuse reflection. Diffuse reflectance measurements are usually made by using a UV-visible spectrophotometer equipped with a diffuse reflectance accessory (integrating sphere) capable of collecting the reflected flux. The Kubelka–Munk and other forms of diffusive reflectance are strictly mathematical artifices with no direct physical meaning. In practice, however, reflectance measurements are performed under specific geometric conditions. Diffuse reflectance spectra are significantly sensitive to the manner in which the soil or mineral mixture sample is prepared. Particle size is the factor most strongly affecting reflectance, as shown by the occasionally dramatic changes in soil color upon grinding.